Lets have look for paytm useful hacks .
You cant transfer the cashback u got for those purchases to bank, so transfer the cash to other paytm account (like your secondary number /your family or friends account) and transfer back to your main number and now you can send that amount to bank just like normal paytm balance
If u could not to take a screenshot after payment success, you can find that transaction details in passbook
Paytm has sunday market, which provides some good deals at low prices,